docs: def state(self, states=None):
          """Filter by state.
  
          :param  tags: States to filter.
          :type   tags: ``list``
  
          :return: A list of Node objects.
          :rtype:  ``list`` of :class:`Node`
          """
          if states is None or not states:
              return self
          nodes = []
          for node in self.nodes:
              if any(state.lower() == node.state.lower() for state in states):
                  nodes.append(node)
          self.nodes = nodes
          return self